Niobium-based oxidation resistant materials and process for their preparation

ABSTRACT

Niobium and niobium-based alloys are protected against oxidation by providing them with a protective coating of mixed metallic oxides of the Beta -Nb2O5 type.

United States Patent [191 Chevillon et al.

[ NIOBIUM-BASED OXIDATION RESISTANT MATERIALS AND PROCESS FOR THEIRPREPARATION [75] Inventors: Jack Rene Paul Chevillon,

Ris-Orangis; Georges Maurice Celestin Alois Gauje, Saulx-les-Chartreux;Jacques Leopold Emile Grammagnac, Paris, all of France [73] Assignee:Societe Nationale dEtude & de

Construction de Moteurs dAviation (S.N.E.C.M.A.), Paris, France 22Filed: Aug. 9, 1971 [21] Appl.No.: 169,932

[30] Foreign Application Priority Data Aug. 12, 1970 France 7029740 [52]US. Cl l48/6.3, 117/127, 117/l35.1,

[ Oct. 16, 1973 148/315 [51] Int. Cl. C23f 7/02 [58] Field of Search117/131, 22, 127, 117/135.1;148/6.3

[56] References Cited UNITED STATES PATENTS 3,057,048 10/1962 l-lirakis117/131 3,102,044 8/1963 Joseph 117/22 3,078,554 2/1963 Carlson 117/1313,219,474 11/1965 Priceman et a1 117/131 X Primary ExaminerRalph S.Kendall Attorney-Mason, Fenwick & Lawrence [57] ABSTRACT Niobium andniobium-based alloys are protected against oxidation by providing themwith a protective coating of mixed metallic oxides of the BNb O type.

6 Claims, 5 Drawing Figures PAIENTEUncHs ms 11,765,953

SHEET 2 BF 3 IAcK RENE PAUL. CuawLLoN, Ciemzces WumcaCELEs-vm ALOlSGAuIE. IAc ues Leopow EMILE GRAMMAGNAC e7 ma m -wuiak% ammaNlOBIUM-BASED OXIDATION RESISTANT MATERIALS AND PROCESS FOR THEIRPREPARATION The present invention concerns a procedure for improving thehot oxidation resistance of niobium and niobium based alloys.

The use of niobium and niobium-based alloys poses considerable problemsas regards the possibility of effective protection against oxidation ofthe work-pieces or their surfaces produced using these materials.Attempts have been made in practice to resolve these problems bydepositing on the surface materials, metals, mixtures of metals, ormetallic alloys or compounds which can be made to adhere to theunderlying material by heating in vacuum or in a neutral or reactiveatmosphere at a high temperature. These metals or alloys form a thinprotective layer which to some degree ensures protection of theunderlying material against the oxidation which can develop afterutilisation of the pieces at high temperature.

But it has been recognised, and systematic tests have confirmed, thatthe protection thus achieved is always insufficient, more particularlywhen these pieces are submitted to successive thermal cycles withrelatively slow rise of temperature followed by thermal shock. Thepieces are destroyed always according to the same process known tospecialists under the name of plague and which is made evident by theappearance of swellings on the surface on the said pieces. Theseswellings, distributed at random, result in very rapid destruction ofthe protective layer and a later rapid destruction of the piece itself.

According to French Pat. No. 1,305,861 a process has been proposed ofprotecting pieces made of niobium or niobium alloys, in which a coatingis depositedon the surface of the said pieces and is constituted by ametal or a metalloid capable of giving a refractory substance, theproduct thus obtained is subjected to annealing, and the sintered layerthus formed is oxidized. Among the metals and metalloids which aresuitable, aluminium and its alloys, silicon and its alloys, and chromiumand its alloys have been cited.

This procedure however has the inconvenient feature of not producing asufficiently durable protection for the niobium or niobium alloyworkpieces, particularly as regards successive thermal shocks to whichthey are submitted.

There has also been proposed in French pat. No. 1,438,749 a process ofprotecting pieces made of niobium or niobium alloys containing at least50 percent by wt. of niobium, according to which the said pieces arecoated with a mixture comprising on the one hand silicon, on the otherhand vanadium and/or manganese, and also at least three elements chosenfrom tantalum, niobium, tungsten, molybdenum, chromium, titanium,zirconium, aluminium and boron. After heating under vacuum or in anargon atmosphere, and oxidation for l to 3 hours, the obtained pieceshowever cannot sustain successive thermal shocks for more than 250hours.

The present invention has as its object a solution of the problem of theoxidisability of pieces made of niobium and niobium alloys, and areduction of the disadvantages of the prior art. It thus concerns, asregards new materials, pieces made of niobium or alloys of niobiumcoated, according to known processes, by socalled protective coatingswith a metal or metallic alloy base forming, with the niobium or niobiumalloy underlying it, associations or alloys of niobium silicide oraluminide type characterised in that the superficial zone of theprotective layer contains at least percent by wt. of metallic oxides oftype (Nb, M M M M,,) O (M M M M,,= metals constituting the alloy or theprotection which substitute for niobium in the complex B-form oxide).

The invention is based on the following scientific observations:

the metallic oxides of niobium of the B-Nb O type are thermally stableand the transformation a B is irreversible; certain complex oxides ofniobium of B-(Nb, M M

M M0 0 type form a protective barrier effective at high or lowtemperature against oxygen;

the metallic oxides of niobium of the B-(Nb, M M M M,,) O type can beprepared by oxidising the superficial protective coating withoutdislodging it as long as one operates in conditions where the oxidesformed, in particular the oxides of a-(Nb, M M M M0 0 type transforminto oxides of the B-(Nb, M M M M0 0 type to the same extent as they areformed.

The application of these various observations to the protection ofniobium or of its alloys thus leads to depositing on the surface ofpieces of niobium, or of its alloys, certain elements, themselves known,based on metals or alloys; producing, by heating under vacuum or neutralor reactive atmosphere and according to known processes, theassociations or alloys of niobium with the deposited elements so as toform derivatives of the alloyed niobium silicide or aluminide type; thentreating by oxidation the piece thus obtained in conditions such thatthe superficial layer of silicide or aluminide type thus created istransformed into a metallic oxide of fl-(Nb, M M M M0 0 type. Thisoxidation of the superficial layer should be effected without oxidationof niobium or of the alloy of niobium which is situated below thislayer; this result can be obtained by effecting the oxidation reactionin conditions such that the speed of transformation of metallic oxidesof the a-(Nb, M M M M0 0 type into metallic oxides of the B-(Nb, M M MM,,) O, type is at least equal the speed of formation of the metallicoxides of the a-(Nb, M M M M O type.

This condition can be expressed in another way by saying that during theoxidation reaction the proportion of metallic oxides of a-(M M M0 0 typein the protective layer should always be low.

When the operating conditions indicated above are followed, theprotection surface is oxidised without penetration of oxygen into theniobium or niobium alloy beneath.

The invention thus concerns a procedure for preparation of pieces of newmaterial, as defined above, characterised in that the said pieces,previously treated according to known processes for formation ofsuperficial metallic protective layers, are subjected to an oxidationreaction under operating conditions in which the speed of formation ofthe metallic oxides of B-(Nb, M M Mhd 3, M0 0 type from the metallicoxides of a-(Nb, M M M M,,) O type is greater than the speed offormation of the metallic oxides of a-( Nb, M M M M0 0 type.

This oxidation reaction should be effected while maintaining thematerial, coated with a metallic layer which is in itself known, in airat atmospheric pressure at a temperature between 700 and l,200C for atime exceeding 50 h and, preferably, for 50-300 h according to thecomposition of the layer. Preferably, also, the treatment is conductedin still air, at a temperature between 700 and 900C.

For a good understanding of the above, what is meant by metallic oxidesof type a or ,8 should be explained. It is known that oxidation ofniobium in air, at high temperature, leads to solution of oxygen in theniobium and the simultaneous formation in the surface of oxides of whichthe principal are NbO, NbO Nb O Niobium pentoxide Nb O can occur in twodifferent crystalline forms; one orthorhombic form, a, and onemonoclinic, B. According to the invention, a thin layer of metal alloysis deposited on the surface of the niobium or the niobium alloy to beprotected. For example, a paint containing nickel, titanium, chromium,and aluminium is deposited on the surface of the niobium. By laterheating in vacuum, there is a chemical reaction between these metals andthe underlying niobium with formation of chemical compounds containingniobium. For example, alloyed niobium aluminide will be formed. When,according to the invention, this superficial layer is subjected tooxidation, the atoms of niobium will oxidise as welll as the othermetallic atoms, leading to the formation of various oxides. For example,there can form the a or B form of (Nb, M,, M M M 0 By definition, wehave termed the metallic oxide of B-(Nb, M M M M0 0 type the oxidationstate of this superficial layer, for which the quasitotality of theatoms of niobium is in the form of Nb O and for which the crystallinestructure of the said superficial layer is monoclinic, that is,identical or similar to the crystalline form of B-niobium pentoxide.

According to a preferred mode of procedure, the protective layer ofBtype (Nb, M M M M0 0 oxides is obtained by depositing on the surface ofthe piece made of niobium or niobium alloys 2. powder comprising atleast three of the following elements, Si, Al, Cr, Fe, Ti, Ni and Co,and subjecting the piece thus coated to a controlled oxidation in theconditions given above.

The powder contains, according to one characteristic of the invention,silicon or aluminium, the silicon or aluminium being always associatedwith niobium and chromium, the fourth constituent being chosen among theremaining metals of the list given above, i. e., iron, titanium, nickeland cobalt. The preferred powders are those comprising:

a. a mixture of Si, Cr, Fe,

b. a mixture of Si, Cr, Ti,

c. a mixture of Al, Cr, Ti,

d. a mixture of Al, Cr, Ni, Ti and e. by the mixtures obtained byincorporating cobalt in each of the powders a, b, c and d.

In these powders the quantities of Si or Al are equivalent, the saidquantities being considerably greater than those of Cr, Fe, Ti, Ni andCo. In general one can use by weight:

50 parts of Si or Al 1 part of Cr 1 part of Fe, Ni, Ti and/or Co.

The non-limitative trials below illustrate the invention. The results ofthese trials can be better understood by referring to FIGS. 1 to 5.

FIGS. 1 and 2 relate to modes of heating of the pieces during the testscarried out (the ordinate gives temperature, the abscissa gives time).

FIGS. 3, 4 and 5 show the comparative speed of oxidation of samples,treated according to the invention, or untreated; FIG. 3 gives theresults obtained with isothermal heating; FIGS. 4 and 5 the resultsobtained with cyclic heating. For these figures the ordinate gives thevariation of weight of the sample and the abscissa gives the time.

These tests have been carried out by subjecting the treated or untreatedsamples to thermal cycles; these thermal cycles are of two types: typeA, in which the samples are kept during considerable times at hightemperature, or type C in which the samples are not kept for a long timeat a high constant temperature.

The type A cycle consists in keeping the sample isothermally for alonger or shorter time at the maximum test temperature, which is usuallybetween 1,000C and 1,400C, with the heating to temperature and thecooling to ambient temperature effected rapidly enough, that is in atime between several seconds and several minutes. FIG. 1 showsschematically two cycles of this type, a cycle A of 90 minutes durationand a cycle A of 24 hours duration.

The type C cycle is as follows: a slow rise to the test temperature,holding for a short time and rapid cooling. FIG. 2 shows schematically atype C cycle of 4 hours duration, where the rate of temperature rise is300C per hour approximately.

If the life times are compared of an ordinary sample of niobium,obtained with the two types of test, it is observed that for a samemaximum test temperature, fixed in this case at 1,100C, and in spite ofa holding time equal to percent of the test duration with cycle A, ofminutes duration against only 2 to 3 for cycle C, of 4 hours duration,that is about 30 times shorter duration, the life time is howeverdivided by a factor of the order of 10 to 20 when one passes from cycleA to cycle C.

EXAMPLE 1 FIG. 3 shows the difference between the speed of oxidation ata constant temperature of 700C between Sample 1, which had not undergonethe previous treatment according to the invention, and Sample 2, whichhad undergone a previous treatment of hours at 900C. These two sampleswere protected by a mixture of powder based on silicon-chromium-iron ofabout 0.15 mm thickness. This example is to show the improvementobtained during prolonged maintenance in the dangerous temperature zone.

EXAMPLE 2 FIG. 4 shows the gain in weight of two samples during anoxidation test according to a cycle of type C with a maximum temperatureof 1,100C. The 2 samples had simultaneously received the same protectionby a mixture of powders based on Si-Cr-Fe, but sample number I wastested as it was, while sample number 2 was additionally subjected to acontrolled thermal oxidation and aging treatment of 200 hours at 900C.Final failure, characterised by a considerable fall in weight, is shownaround 80 hours for sample number 1 and at 875 hours for sample number2, which represents an improvement factor of the order of 10.

EXAMPLE 3 Two samples protected by a deposit of type Si-Cr-Fe of about0.15 mm thickness were tested as in the example above, following cycle Cat 1,100C, and gave the following results:

the first non treated sample had a life time of hours;

the second sample, having received a previous controlled thermaloxidation and ageing treatment according to the invention of 240 hoursat 900C in air, at atmospheric pressure, had a life time of 300 hours.

The application of the treatment thus gave, in this case, amultiplication by 30 of the life time.

EXAMPLE 4 FIG. 5 permits examination of the gains in weight obtained inoxidation tests following a type A cycle at 800C with durations of 24hours. The weight gain was obtained by weighing after each period. Thesamples of niobium alloy were protected by a coating of 0.03 mmcomprising chromium, titanium, and aluminium deposited in the vapourphase. Sample number 1 was untreated, but sample number 2 had undergonea controlled thermal oxidation and aging treatment of 264 hours at 700C.A spectacular difference is seen between the two tests, sample number 1being rapidly destroyed and sample number 2 being still intact at theend of 792 hours of testing.

EXAMPLE 5 This example is given to show the importance of not exceeding900C, this temperature constituting the approximate limit above whichthe oxidation rate of niobium and the elements associated with it into aa-type Nb O become prohibitive for the control of the process.

Two sets of test pieces were coated using a powder of Si, Cr, Fe. Ineach set the test pieces were either subject to a preliminary oxidationat atmospheric pressure in still air, or were not subject to the saidoxidation. The life time according to cycle C is shown in Table I below.

TABLE 1 RESULTS OF TESTING WITH TWO SETS OF TEST PIECES PROTECTED BYSYLVANIA USING A DEPOSIT OF Si-Cr-Fe Preliminary Oxidation Life time (h)(Atmospheric Pressure) in cycle C at ll00C Set. medium: still air No.untreated 60 1 untreated I30 1 3 h at ll00C 178 l 15hat 1100C 210 l 195h at 900C 875 l 195 h at 900C 905 1 untreated 8 2 untreated 12 2 237 hat ll00C 300 2 We claim:

1. Niobium or a niobium-based alloy having a protective coating formedof mixed metalic oxides of the B-(Nb, Me) O type, wherein Me includes ametal having a major amount of aluminum or silicon alloyed with chromiumand at least one other metal taken from the group consisting essentiallyof iron, titanium, nickel, cobalt, or mixtures thereof.

2. Niobium or a niobium-based alloy according to claim 1 wherein saidprotective coating comprises at least percent by weight of mixedmetallic oxides of the Beta form. c

3. Niobium or a niobium-based alloy according to claim 2 are 50 partsaluminum and/or silicon to 1 part each of chromium, iron, nickel,titanium and/or cobalt.

4. Niobium or a niobium-based alloy according to claim 1, wherein theprotective coating composition is taken from the group comprising thefollowing combination of metals:

Si, Cr, Fe;

Si, Cr, Ti;

Al, Cr, Ti;

Si, Cr, Fe, Co;

Si, Cr, Ti, Co;

Al, Cr, Ti, Co; or

Al, Cr, Ni, Ti, Co.

5. Process for the manufacture of niobium or a niobium-based alloycomprising the steps of forming a protective coating of the (Nb, Me) Otype wherein Me includes a metal having a major amount of aluminum orsilicon alloyed with chromium and at least one other metal taken fromthe group consisting essentially of iron, titanium, nickel, cobalt, ormixtures thereof, heating and oxidizing said (Nb, Me),O layer at 700 to1,200C for a time sufficient to convert a major portion of said layer tothe B-oxide form.

6. The process according to claim 5 wherein said protective layer isheated in air at 700C to 900C for at least 50 hours.

2. Niobium or a niobium-based alloy according to claim 1 wherein saidprotective coating comprises at least 70 percent by weight of mixedmetallic oxides of the Beta form.
 3. Niobium or a niobium-based alloyaccording to claim 2 are 50 parts aluminum and/or silicon to 1 part eachof chromium, iron, nickel, titanium and/or cobalt.
 4. Niobium or aniobium-based alloy according to claim 1, wherein the protective coatingcomposition is taken from the group comprising the following combinationof metals: Si, Cr, Fe; Si, Cr, Ti; Al, Cr, Ti; Si, Cr, Fe, Co; Si, Cr,Ti, Co; Al, Cr, Ti, Co; or Al, Cr, Ni, Ti, Co.
 5. Process for themanufacture of niobium or a niobium-based alloy comprising the steps offorming a protective coating of the (Nb, Me)2 O5 type wherein Meincludes a metal having a major amount of aluminum or silicon alloyedwith chromium and at least one other metal taken from the groupconsisting essentially of iron, titanium, nickel, cobalt, or mixturesthereof; heating and oxidizing said (Nb, Me)2O5 layer at 700* to 1,200*Cfor a time sufficient to convert a major portion of said layer to theBeta -oxide form.
 6. The process according to claim 5 wherein saidprotective layer is heated in air at 700*C to 900*C for at least 50hours.